1. Field of the Invention
This invention pertains to an improved, faster and more reliable method for generating random pattern apertures in a master suitable for manufacturing light shaping diffusers and similar optical components.
2. Discussion of the Related Art
In the past, coherent lasers have been used to manufacture optical products such as light shaping diffusers. As shown in FIG. 1, a photosensitive medium 4, such as photoresist, is recorded by exposing it to coherent (laser) light 1 from a krypton laser passed through a spatial filter 2 and a diffuser 3. The diffuser 3 may be a ground glass, holographic, lenticular or acetate diffuser, or a diffuser itself previously recorded in the recording set-up of FIG. 1.
Preferred methods and apparatuses for making such diffusers are disclosed in U.S. Pat. No. 5,365,354 entitled "Grin Type Diffuser Based on Volume Holographic Material," U.S. Pat. No. 5,534,386 entitled "Homogenizer Formed Using Coherent Light and a Holographic Diffuser," and U.S. Pat. No. 5,609,939 entitled "Viewing Screen Formed Using Coherent Light," all owned by the present assignee, relate to methods for recording optical products such as diffusers and replicating those diffusers so that they may be mass produced. Each of these U.S. patents is incorporated by reference herein for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art. Related U.S. patent applications include Ser. No. 08/595,307 entitled "LCD With Light Source Destructuring and Shaping Device," Ser. No. 08/601,133 entitled "Liquid Crystal Display System with Collimated Backlighting and Non-Lambertian Diffusing," Ser. No. 08/618,539 entitled "Method of Making Liquid Crystal Display System," Ser. No. 08/800,872 entitled "Method of Making Replicas and Compositions for Use Therewith," and Ser. No. 09/052,586 entitled "Method of Making Replicas While Preserving Master." All the above applications are owned by the present assignee and are hereby incorporated by reference for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art.
The methods taught in these patents produce both internal and surface structures in the photosensitive medium 4 called "speckle" which diffuse light in a highly efficient, uniform, and controlled manner not possible with prior methods. As fully described in the above patents, the size and shape of the speckle recorded in the photosensitive medium can be controlled, thereby controlling the angular output of light from the diffuser after it is developed. Diffusers made by the methods taught in the above patents are extremely useful as viewing screens and homogenizers, both of which can be used in myriad applications.
For mass production, the surface structures which remain in the photosensitive medium 4 after processing are exploited. After the photosensitive medium has been exposed for a suitable length of time, it is processed to make a master. A first generation submaster or replica made of epoxy or other plastic resin may then be made from the master by applying epoxy to the surface of the master, uniformly spreading the epoxy out on the master, and then separating the epoxy from the master after the epoxy has been cured. Successive generations of submasters are typically made from the previous generation submaster using the above process. Each successive generation submaster exhibits a change (usually a reduction) in the aspect ratio of the surface structure features due to shrinkage.
The prior art processes outlined above have a number of practical shortcomings. First, the overall size of diffusers capable of being produced is limited by the intensity of the lasers available and the sensitivity of the photosensitive media. For example, prior art systems typically require a coherent light source capable of providing very high energy density on the order of 2.7 joules/cm.sup.2 to generate a suitable exposure. As a result, in the past, large masters have been assembled from a number of smaller submasters placed next to one another in an attempt to make a larger, seamless master as shown in FIG. 2. With this approach however, it has been difficult to avoid discontinuities and seams from appearing along the edges where the submasters are joined.
Another problem encountered in making large master diffusers has been the appearance of an offset of the apertures in the corners relative to those in the central region, resulting in undesirable non-uniform patterns. Finally, because of the relatively slow reaction of photoresist to light, and the physical separation during recording required among the light source, the diffuser and the photosensitive media as seen in FIG. 1, prior art systems are known to be extremely sensitive to vibration and movement. Even the slightest vibration may create phase changes in the coherent light source and thus cause undesirable aberrations in the master. The size of these aberrations can exceed the size of the speckle, rendering the master useless. Vibration and other shortcomings of the above recording methods make it difficult if not impossible to record speckle of extremely small size. In order, for instance, to make a diffuser having an extremely wide output in the horizontal direction, and an extremely narrow output in the vertical direction, the speckle recorded in the photosensitive medium must be very small in the horizontal direction (and large in the vertical direction). (Light output orientation from the diffuser is inversely proportional to speckle size and orientation within the diffuser.) For example, in order to triple the horizontal output angle, speckle size must be decreased to one third.
Furthermore, in the prior art, a separate master had to be made for each diffuser having a particular angular output, thus necessitating a large library of masters having different angular outputs. For instance, a different master is required to achieve a 10.degree..times.10.degree. circular output, 10.degree..times.15.degree. elliptical output and so on. In order to create each of these masters, the recording set-up of FIG. 1 had to be employed. As discussed above, this recording process is slow and susceptible to vibration and other performance degrading factors.
A method of making large, seamless masters that is not susceptible to vibration and is faster and less expensive would be of great benefit.